1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission.
2. Description of Related Art
An automatic transmission inclusive of a continuously variable transmission is generally designed so as to determine a target speed ratio or shift range (collectively referred to as “speed ratio”, hereinafter) based on the vehicle speed and the engine load, and to carry out shift control to hold or change the current speed ratio. Thus, when the vehicle speed sensor for detecting the vehicle speed encounters trouble due to failure, etc., an actual vehicle speed cannot be correctly detected and the detected vehicle speed value different from the actual vehicle speed may be used as one of the control parameters, thereby making it difficult or impossible to carry out a proper shift control.
In order to eliminate such a problem, a generally conceivable solution may be to perform shift control of the transmission based on a fixed vehicle speed at an intermediate level, e.g., 50 km/h, instead of incorrectly detected vehicle speed, assuming that a fixed vehicle speed of such a level does not hinder a steady running of the vehicle on ordinary road. However, it is considered that such a solution results in further problem.
That is to say, with the above-mentioned solution, since shift control is based on a fixed vehicle speed value at an intermediate level even in a stopped state of the vehicle, there may be caused so-called “high-starting” of the vehicle, wherein the accelerating performance of the vehicle deteriorates due to insufficient torque. This is because the fixed vehicle speed value at an intermediate level corresponds to a target speed ratio that is on a higher speed side in relation to the stopped state of the vehicle, and not to the speed ratio for the lowest-speed range required when actually starting the vehicle.